1. Field of the Invention
This invention relates to a device having a movable belt and, more particularly, to an image forming apparatus such as an electrophotography apparatus or laser beam printer having a movable belt and capable of recording image information on an image support member such as a transfer material.
2. Related Background Art
Conventional color electrophotography copiers based on electrophotography using a plurality of image formation processing steps to form a color image make use of several types of systems a typical one of which is illustrated in FIG. 5.
A color electrophotography copier shown in FIG. 5 has four image forming stations Pa to Pd each of which has rotary photosensitive drums 1a to 1d provided as image bearing members. Around the photosensitive drums 1a to 1d are respectively disposed charging sections 2a to 2d, exposure sections 3a to 3d, development sections 4a to 4d, transfer sections 5a to 5d and cleaning sections 6a to 6d in the direction of rotation of the drums.
A transfer belt means including an endless belt, i.e., a transportation means 7 is disposed below the photosensitive drums 1a to 1d so as to pass through the image forming stations Pa to Pd. The transportation belt means 7 transports, through the transfer sections 5a to 5d of the image forming stations Pa to Pd, a transfer sheet 9 supplied by paper feed rollers 8 disposed at its one end.
Formation of a color image with the thus-constructed color electrophotography copier will be described below. A latent image of an original image in a yellow component color is formed on the photosensitive drum 1a by using the charging section 2a and exposure section 3a of the first image forming station Pa, i.e., by a well-known electrophotography means, this latent image is changed into a visible image at the development section 3a by a developer containing a yellow toner, and, at the transfer section 4a, the yellow toner image thereby made visible is transferred to the transfer sheet 9 transported by the transportation belt means 7.
During transfer of the yellow toner image to the transfer sheet 9, a latent image of the original image in a magenta component color is formed on the photosensitive drum 1b in the second image forming station Pb in the same manner as the yellow toner image, and a magenta toner image is obtained at the development section 4b by using a magenta toner. When the transfer sheet 9 to which the yellow toner image has been transferred in the first image forming station Pa is transported to the transfer section 5b of the second image forming station Pb, the magenta toner image is transferred to the transfer sheet 9 at a predetermined position.
With respect to cyan and black colors, image formation is effected in the same manner. After superposition of the four color toner images on the transfer sheet 9 has been completed, the transfer sheet 9 is transported to a fixation section 10 disposed at the other end of the transportation belt means 7, and the color images are fixed in this section, thereby obtaining a multi-color (full-color) image on the transfer sheet 9.
After transfer, an amount of toner remaining on each of sensitive drums 1a to 1d is removed by the corresponding one of the cleaning means 6a to 6d, thereby preparing each drum for the subsequent latent image formation.
This type of full-color image forming apparatus has the following advantages.
(1) It has independent stations for forming images in respective colors, and is therefore effective for speedup of image formation. PA1 (2) The transfer path can be formed along a straight line and the apparatus is therefore applicable to formation of an image even on a thick sheet of paper or a transparent film.
However, this type of image forming apparatus entails a most serious drawback relating to how the desired performance of registration of the color images formed in the different image forming stations is achieved. Offsets between the positions at which the four color images are formed by transfer to the transfer sheet (hereinafter referred to as "registration offset") finally appear as color offsets or changes in hues. One of the causes of such a registration offset relates to a phenomenon of one-sided movement of the transportation belt of the transportation belt means 7 (transportation belt 100 (FIG. 6)) for transferring the transfer sheet as a result of failure to effect a straight-line motion, i.e., a phenomenon of meandering of the transportation belt 100 or deviation of the same from a predetermined course on the rollers for rotating, retaining and guiding the transportation belt 100.
A means for correcting such an offset of the transfer belt, i.e., the transportation belt 100 is known which includes a guide groove formed in each of guide rollers which rotate, retain and guide the transportation belt, and a guide rib formed on the transportation belt at the position corresponding to the guide groove.
This kind of means which essentially consists of guide grooves in the guide rollers and a guide rib on the transportation belt and which is thus provided as a simple mechanism can limit the offset of the transportation belt and is also advantageous in terms of durability because, even though the thickness of transportation belt is small, the thrust force is applied to the end surface of the guide rib improved in strength.
The color electrophotography copier shown in FIG. 5 is also provided with a guide rib 101A formed at one end of a belt base fabric 100A, and guide grooves 11A to 14A formed in transportation belt guide rollers 12 to 14 and in a transportation belt drive roller 11 provided to drive the transportation belt 100, the guide grooves being formed at predetermined positions corresponding to the guide rib 101A on the belt base fabric 100A, as shown in FIG. 6. (Guide grooves 13A and 14A are not illustrated.) The guide rib 101A is fitted in these guide grooves, thereby determining the path for the transportation belt while maintaining the linearity of the movement of the transportation belt.
In the conventional offset correcting means or transportation belt meandering correcting means including rollers having guide grooves and a belt having a guide rib, the surface of the drive roller is formed from a material having a comparatively large friction coefficient .mu., e.g., rubber in order to improve the efficiency with which the driving force is transmitted to the transportation belt. Also, a thin belt is adopted as the transportation/transfer belt in consideration of the performance of transfer and adhesion to the transfer sheet, and the guide rib fixed to the belt is formed of a rubber having a smaller rigidity in order to avoid any considerable influence on the flexibility of the transportation belt. Therefore the material of the guide rib has a friction coefficient .mu. as large as that of the surface of the drive roller.
If a belt having such a guide rib is wound around the rollers and is moved by the drive roller, the side surface of the guide rib slides on the mating side surface of the guide groove of each roller to prevent the transportation belt from meandering. It is possible that a portion of the side surface of the guide rib may be pressed against the side surface of one of the guide grooves. If this state continues for a long period of time, a large frictional resistance occurs between the side surfaces of the guide rib and the guide groove, because both friction coefficients of the rubber forming the surface of the roller and the rubber forming the guide rib are large. The desired sliding performance of the guide rib is thereby impaired, and the speed at which the transportation belt is moved for transportation is thereby changed. If the side surface of the guide rib further continues sliding on the side surface of the guide groove, there is a possibility of the guide rib riding on the surface of the roller, thereby impairing the linearity of the movement of the transportation belt.
This phenomenon causes a reduction in the area of contact between the transportation belt and the drive roller and allows flying toner to attach to the surface of the roller so as to reduce the friction coefficient of this surface. Transmission of the driving force to the transportation belt is thereby made considerably unstable. In consequence, a registration offset takes place and causes blur of a color or a change in a hue, resulting in a deterioration in the qualities of the image.
To stop the guide rib from floating from the roller, the tension applied to the transportation belt may be increased. However, if the tension is excessively large and if the guide rib and the transportation belt are formed of resins, a creep strain may be caused such that portions of the guide rib and the transportation belt which are in contact with the rollers during stoppage of the apparatus are deformed, resulting in failure to perform a smooth transportation movement of the transportation belt. Moreover, because the elogation of the portion of the transportation belt on which the guide rib is fixed is different from that of other portion, an excessive stress is caused in the portion to which the guide rib is fixed, resulting in a reduction in the lifetime of the transportation belt.